This project is a collaboration with Prof. Rolf Ziegler of the Institute for Zoology, Martin Luther University, Halle-Wittenberg, Germany, and of the Department of Biochemistry and Center for Insect Science at the University of Arizona, Tucson. The work has been supported by NIH grants AI 26905 and GM 29238, and has resulted in two collaborative publications with the NTLF. This research furthers the understanding of the interactions between ligands and their receptors, specifically as applied to insects. It examines the regulation of energy metabolism in Manduca sexta insects by adipokinetic hormone (M-AKH). Peptides of the AKH family control the mobilization of energy stores in times of need. In molting and in wandering larvae of M. sexta AKH mobilizes fat body lipids and carbohydrates, the latter by activating glycogen phosphorylase, in similarity with glucagon stores in mammals. In initial work, the tritium labelled M. sexta M-AKH (pGlu-Leu-Thr-p-3H-Phe-Thr-Ser-Ser-Trp-Gly-NH2) was synthesized at very high specific activity, and was fully active in a bioassay. It was used in a filtration based binding assay to characterize the M-AKH from the fat body of M. sexta. Recently, single amino acid replacement analogs of M-AKH were tested for activity in receptor binding assays. Amino acids were replaced by Ala and by D-analogs. In addition, an extended M-AKH and analogs containing photoaffinity labels were tested. All analogs had reduced activity. All the peptides that had enough activity to allow a full dose response curve reached the same maximal activity as native M-AKH. The use of analogs, in which L-Phe4 was replaced by Ala or by D-Phe and L-Thr3 replaced by D-Thr, as competitors led to improved binding of M-AKH in our competitive receptor binding assays. In the bioassay an inactive concentration of Ala4 M-AKH increased the activity of a half optimal conc entration of native M-AKH. The well-characterized tritium labelled M-AKH of high specific activity is central to these studies.